Optical communications has undergone very rapid development, to the point where optical fiber transmission systems are becoming almost commonplace. Optical fiber is not only used for trunk (i.e., long-haul) applications, but also to transfer information over short distances, e.g., in local area networks, and potentially, in the subscriber loop.
In many applications it is necessary, or at least desirable, to inject optical power into a fiberguide at intermediate locations, and/or to extract optical power from the fiberguide at such locations, without the need for breaking or terminating the fiberguide, and without requiring special preparation of the coupling point in the fiberguide. Such couplers have been known in the art for some time. See, for instance, co-assigned U.S. Pat. No. 3,931,518, ('518), issued Jan. 6, 1976, to S. E. Miller, which teaches a particular embodiment of a coupler type which will herein be referred to as a "grating" coupler.
The '518 patent teaches that optical power can be coupled from an optical fiber by impressing a periodic deformation onto the fiber, with the periodicity of the deformation chosen such as to induce coupling between appropriate modes of radiation. In this fashion power can be transferred resonantly from lower to higher order guided or bound modes, and nonresonantly from the higher bound modes to the so-called tunneling leaky (TL) modes, which are then removed from the cladding of the fiberguide with the aid of a dielectric body that is in contact with the fiberguide at a point downstream from the periodic deformation region and which has a refractive index which is approximately equal to or greater than the index of refraction of the cladding.
For an exposition of the relevant theory, see, for instance, D. Marcuse, Theory of Dielectric Optical Waveguides, Academic Press, 1974, especially pages 95-157. Briefly, it can be shown that it is possible to provide a coupling mechanism in multimode fiber such that the (i, j)'th and (p, q)'th bound modes are coupled to produce complete energy exchange over a coupling length L.sub.c =.pi./R.sub.ij,pq, where the coupling constant R.sub.ij,pq depends upon fiber parameters such as the core radius, the refractive index difference between core and cladding, the operating wavelength, the fiber profile shape, and, in a coupler as disclosed in '518, on the amplitude of the distortion of the fiber. As the distortion amplitude increases, the coupling length L.sub.c decreases. Thus, a prior art coupler as disclosed in '518 can be tuned for maximum efficiency by adjusting the amplitude of the distortions of the multimode fiber, to result in resonant energy transfer from low to higher order bound modes.
Although '518 teaches that optical power can be coupled from the single guided mode (usually referred to as the LP.sub.01 mode) of single mode fiber to one or more of the TL modes of such fiber, and that, therefore, couplers of the type disclosed in '518 could be used not only with multimode fiber but also with single mode fiber, this type of coupler has in fact only been used in conjunction with multimode fibers. The reason for this is as follows. It is generally understood in the art that the coupling process between bound modes in multimode fiber is a resonance process, and that consequently the coupling parameters can be adjusted to result in efficient resonant power transfer into high order bound modes, and from there nonresonantly into TL modes. On the other hand, the theory teaches that, in single mode fiber, the coupling between LP.sub.01, the bound mode, and a TL mode, e.g., LP.sub.11, is nonresonant, such that the radiation amplitude in LP.sub.01 decreases exponentially with distance along the propagation direction, due to the continuous transfer to the TL mode of a constant fraction of the power in LP.sub.01. See, for instance, page 112 of the above cited book by Marcuse, where it is stated that, for the case of a single-mode guide, the power coupled into TL (radiation) modes is radiated from the guide and does not interact with the guided mode. Since such nonresonant coupling cannot be tuned to result in efficient power transfer between LP.sub.01 and TL modes, it is generally accepted in the art that fiber taps of the "grating" type cannot be made to function efficiently in single mode fiber.
The fact that microbending-induced mode coupling in multimode fibers can involve a resonance mechanism has also been used to construct highly sensitive fiber optic displacement sensors. See, for instance, N. Lagakos, Digest of Technical Papers of the Conference on Optical Fiber Communication, New Orleans, La., January 1984, pp. 56-58.
G. F. Lipscomb et al, First International Conference on Optical Fiber Sensors, London, April 1983, pp. 117-121, report on the result of experiments with single mode and multimode optical fiber, in which a single bend was induced in the fiber by bending the latter around a cylindrical mandrel. Interference effects between bound modes and TL modes were observed in both types of fiber. In particular, it was observed that the bending causes some of the core-mode power to convert into the cladding-mode power and, at specific angles, some of the cladding-mode power to convert back into core-mode power. It will be noted that the interference effects in the single-bend configuration of Lipscomb et al are not the desired resonance coupling effects that are of concern in this application. In this respect, see also pages 156-157 of the above cited book by Marcuse.
K. P. Jackson et al Applied Physics Letters, Vol. 41(2), pp. 139-141 (1982) report on a tapped single mode optical fiber delay line. The taps were formed by urging a tapping pin against the fiber, thereby inducing a 1.5 mm bend radius in the fiber. No resonant coupling is involved in this technique.
Since single mode optical fiber is rapidly becoming the fiber type of choice for long distance transmission, and is considered to be a promising medium even for short-haul applications in which a multiplicity of sending and/or receiving stations are connected by a single or dual fiber transmission path, it is clear that it would be very desirable to have available efficient means for coupling optical power into, and/or out of, single mode optical fiber without breaking the fiber and without permanently changing the characteristics of the fiber in the coupling region. This application discloses such coupling means.